(1) Technical Field
The present invention relates to a sub-millimeter wave frequency heterodyne imaging system. More specifically, the present invention relates to a sub-millimeter wave frequency heterodyne detector system for imaging the magnitude and phase of transmitted power through or reflected power off of mechanically scanned samples at sub-millimeter wave frequencies.
(2) Background
A wealth of useful spectral information exists at sub-millimeter wave frequencies: 300 GHz-3000 GHz (wavelengths from 1 millimeter (mm) to 100 microns). Astronomers have long pursued programs to develop this technology for Earth, planetary and space science applications; and recently, the biological and biomedical areas have begun pursuing the same technology. However, the same properties that make the sub-millimeter wave frequencies so interesting—high absorption and emission from many gaseous species, liquids and solids (especially water)—make it extremely difficult for significant penetration or propagation of terahertz energy, thus severely limiting imaging, radar and communications applications.
One current method of examining biological and other material samples is through the use of ultra-fast pulse time-domain spectroscopy (TDS). This technique yields wide spectral coverage, but has limited frequency resolution and signal-to-noise ratio. TDS also has limited penetration depth in lossy samples such as wet tissue, where measured penetration depth is generally on the order of a few microns. For any transmission application, the losses due to water absorption are untenably high; measured at an absorption coefficient of 450 cm−1 at 2.5 terahertz. Thus, the applicability of TDS to biological materials is limited.
Additionally, the Fourier Transform Spectroscopy (FTS) method has also been implemented for measuring biological materials. While FTS is more sensitive and has a much shorter sampling time than traditional spectroscopy techniques, it is again limited in its dynamic range and spectral resolution. For applicability to biological materials, it is similar to TDS—possessing a penetration depth of only microns in wet tissue.
Therefore, a need exists in the art for a sub-millimeter wave frequency detector system with increased dynamic range and measurement capability. Specifically, an imaging system that can measure weak signals through very lossy samples is desirable.